Mass flow meters/controllers and methods having improved accuracy

ABSTRACT

An example method to perform optical measurements involves: emitting a first light beam via a first light source; performing first measurements by detecting the first light beam via a first optical sensor; emitting a second light beam via a second light source; performing second measurements by detecting the second light beam via a second optical sensor, the first and second measurements comprising variable components; performing third measurements by detecting a third light beam emitted from the second light source via a third optical sensor, the third measurements comprising a first steady state component representative of light intensities of the first and second light sources; and compensating a first light output of the first light beam and a second light output of the second light beam by controlling one or more currents to the first and second light sources based on the first steady state component of the third measurements.

BACKGROUND

This disclosure relates generally to mass flow measurement and controland, more particularly, to mass flow meters/controllers and methodshaving improved accuracy.

Coriolis effect-based mass flow meters measure mass flow of media bydetermining a phase difference between different portions of a flow tubethrough which the media flows.

SUMMARY

Mass flow meters/controllers having improved accuracy, substantially asillustrated by and described in connection with at least one of thefigures, as set forth more completely in the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of an example mass flow meter/controller,in accordance with aspects of this disclosure.

FIG. 2 is a circuit diagram of an example implementation of the opticalsensors, the light sources, and the compensation circuit of FIG. 1 .

FIG. 3 is a circuit diagram of another example implementation of theoptical sensors and compensation circuit of FIG. 1 , including a thirdoptical sensor coupled to one of the light sources.

FIG. 4 is a circuit diagram of another example implementation of theoptical sensors and compensation circuit of FIG. 1 , includingadditional optical sensors coupled to each of the light sources.

FIG. 5 illustrates an example implementation of a light source having anoptical sensor coupled to receive light output by the light source

FIG. 6 is a flowchart representative of an example method that may beperformed by the mass flow meter/controllers of FIGS. 1-4 to compensateoptical sensors and/or light sources for changes in temperature and/orother effects.

The figures are not necessarily to scale. Where appropriate, similar oridentical reference numbers are used to refer to similar or identicalcomponents.

DETAILED DESCRIPTION

For Coriolis-effect flow meters, optical sensor temperature stability isa factor affecting the measurement accuracy. Each optical sensor, suchas a photodiode or a phototransistor, involves a light source, such as alight emitting diode (LED). Both LEDs and optical sensors havecharacteristics that are at least partially dependent on ambienttemperature. For example, luminance intensity of an LED may vary withtemperature while excited using the same excitation current.

Conventional techniques for temperature compensation using for LEDs andoptical sensors include operating the LED under constant current,operating LEDs under constant temperature, biasing the LED usingtemperature compensation circuits, and compensating the optical sensoroutput for temperature. Furthermore, optical sensors trend towardreduced sensitivity and LEDs toward reduced luminance intensity overlong periods of time.

Constant temperature operation involves additional components such as aheater/cooler, power sources, and control circuits. Such techniques arenot acceptable for low power devices, power sensitive devices, and/orspace-limited devices. Conventional temperature compensation circuitsrequire a temperature monitor and a processor executing compensationalgorithms. Because luminance intensity is a nonlinear function oftemperature, conventional temperature compensation circuits do not fullycompensate for changes in temperature.

Disclosed example systems and methods decouple the light intensity ofthe light source from the temperature of the light sources, providing astable source of light for measurement by the optical sensors. In someexamples, a Coriolis-effect flow meter includes a third optical sensorin addition to two other optical sensors used for the phase/timedifference measurements of the Coriolis-effect flow meter. The thirdoptical sensor is not engaged with vibrating tube. As a result, theoutput of the third optical sensor is not modulated by the flow tube,and is proportional only to the emitted luminance of the light beam.

In some disclosed systems and methods, a third optical sensor lightemitter (LED) is connected in series with the two other optical sensorlight emitters used to generate phase/time difference measurements. Indisclosed examples, the excitation current for all of the light sourcesof the optical sensors is identical and under closed-loop control. Theexcitation current is controlled to stabilized light intensity from thelight sources regardless of temperature, aging, and/or any otherdisturbances in the circuits. All three optical sensors may be locatedclose to each other such that a temperature difference between alloptical sensors is reduced or minimized.

In some examples, the stability of light intensity depends primarily ona stability of a reference signal and total control loop gain value. Anychanges in the light intensity due to a temperature variation orcomponents edging are compensated by the control loop. Since all threeoptical sensor LEDs are connected in series, the light stabilization forthe third sensor provides light stabilization for two other opticalsensors.

In some disclosed examples, a second optical sensor is added to receivelight from one the light sources, and provides a closed-loop controlsignal for luminance intensity stabilization of the light sources. Insuch examples, the number of light sources may be reduced to two. Insome examples, additional optical sensors are added to both lightsources, and the outputs of the optical sensors is averaged, combined,or otherwise filtered to provide feedback for luminance stabilization.Two outputs averaged signal provides better accuracy of compensationsince both optical sensor variations are included into control loop.

By improving temperature stability, disclosed systems and methodssubstantially improve metrological characteristics of Coriolis-effectflow meters, which depend on stability and accuracy of optical sensors.By improving temperature stability of the light sources and opticalsensors, disclosed systems and methods improve overall precision of theflow meter.

As used herein, the term “substantially constant output” of a lightsource refers to an output that has a degree of consistency equal to orbetter than a degree of consistency of a reference signal used tocontrol the output.

Disclosed example optical measurement systems include: a first lightsource configured to emit a first light beam; a first optical sensorconfigured to output first measurements based on detecting the firstlight beam; a second light source configured to emit a second lightbeam; a second optical sensor configured to output second measurementsbased on detecting the second light beam, wherein the first measurementsand the second measurements comprise variable components; a thirdoptical sensor configured to output third measurements based ondetecting the second light beam or a third light beam, wherein the thirdmeasurements comprise a first steady state component; and a compensationcircuit configured to control a first light output of the first lightbeam and a second light output of the second light beam by controllingone or more currents to the first light source and the second lightsource based on the first steady state component of the thirdmeasurements.

Some example optical measurement systems further include: a flow tubeconfigured to direct a fluid from an inlet of the flow tube to an outletof the flow tube; and an actuator configured to cause a vibration in theflow tube, in which a first variable component of the first measurementsis based on the vibration at a first location on the flow tube, and asecond variable component of the second measurements is based on thevibration at a second location on the flow tube.

In some example optical measurement systems, the first optical sensor isconfigured to output the first measurements of a first position of thefirst location on the flow tube based on detecting the first light beam,and the second optical sensor is configured to output the secondmeasurements of a second position of the second location on the flowtube based on detecting the second light beam. Some example opticalmeasurement systems further include control circuitry configured todetermine at least one of a mass flow rate through the flow tube or adensity of the fluid in the flow tube based on the first measurementsand the second measurements. In some example optical measurementsystems, the first location on the flow tube is positioned at leastpartially between the first light source and the first optical sensor,and the second location on the flow tube is positioned at leastpartially between the second light source and the second optical sensor.

Some example optical measurement systems further include a fourthoptical sensor configured to output fourth measurements based ondetecting the first light beam or a fourth light beam, in which thefourth measurements include a second steady state component, and inwhich the compensation circuit includes a filter circuit configured tofilter the first and second steady state components, and to control theone or more currents to the first light source and the second lightsource based on the filtered first and second steady state components.

In some example optical measurement systems, the compensation circuit isconfigured to control the first light source and the second light sourceto output a substantially constant output over a range of temperaturesof the first light source and the second light source. In some exampleoptical measurement systems, the first light source includes a firstlight emitting diode (LED) and the second light source comprises asecond LED. In some example optical measurement systems, the first LEDand the second LED are coupled in series and have a same excitationcurrent, wherein the compensation circuit is configured to control theexcitation current.

In some example optical measurement systems, the compensation circuit isconfigured to compare the third measurements to a reference, and controlthe one or more currents based on the comparison. In some exampleoptical measurement systems, the first, second, and third opticalsensors are thermally coupled. Some example optical measurement systemsfurther include a third light source configured to output the thirdlight beam, wherein the third optical sensor configured to output thethird measurements based on detecting the third light beam.

Other disclosed example optical measurement systems include: a firstlight source configured to emit a first light beam; a first opticalsensor configured to output first measurements based on detecting thefirst light beam; a second light source configured to emit a secondlight beam; a second optical sensor configured to output secondmeasurements based on detecting the second light beam, wherein the firstmeasurements and the second measurements comprise variable components; athird light source configured to emit a third light beam; a thirdoptical sensor configured to output third measurements based ondetecting the third light beam, wherein the third measurements comprisea first steady state component; and a compensation circuit configured tocontrol a first light output of the first light beam and a second lightoutput of the second light beam by controlling one or more currents tothe first light source and the second light source based on the firststeady state component of the third measurements.

In some example optical measurement systems, the first light sourceincludes a first light emitting diode (LED) and the second light sourceincludes a second LED. In some example optical measurement systems, thefirst LED, the second LED, and the third LED are coupled in series andhave a same excitation current, wherein the compensation circuit isconfigured to control the excitation current. In some example opticalmeasurement systems, the compensation circuit is configured to comparethe third measurements to a reference, and control the one or morecurrents based on the comparison.

Some example optical measurement systems further include: a flow tubeconfigured to direct a fluid from an inlet of the flow tube to an outletof the flow tube; and an actuator configured to cause a vibration in theflow tube, in which a first variable component of the first measurementsis based on the vibration at a first location on the flow tube, and asecond variable component of the second measurements is based on thevibration at a second location on the flow tube. In some example opticalmeasurement systems, the first optical sensor is configured to outputthe first measurements of a first position of the first location on theflow tube based on detecting the first light beam, and the secondoptical sensor is configured to output the second measurements of asecond position of the second location on the flow tube based ondetecting the second light beam. In some example optical measurementsystems, the first location on the flow tube is positioned at leastpartially between the first light source and the first optical sensor,and the second location on the flow tube is positioned at leastpartially between the second light source and the second optical sensor.

Disclosed example methods to perform optical measurements involve:emitting a first light beam via a first light source; performing firstmeasurements by detecting the first light beam via a first opticalsensor; emitting a second light beam via a second light source;performing second measurements by detecting the second light beam via asecond optical sensor, wherein the first measurements and the secondmeasurements comprise variable components; performing third measurementsby detecting the second light beam or a third light beam via a thirdoptical sensor, wherein the third measurements comprise a first steadystate component; and compensating a first light output of the firstlight beam and a second light output of the second light beam bycontrolling one or more currents to the first light source and thesecond light source based on the first steady state component of thethird measurements.

Some example methods involve: directing fluid from an inlet of a flowtube to an outlet of the flow tube; causing a vibration in the flowtube, wherein a first variable component of the first measurements isbased on the vibration at a first location on the flow tube, and asecond variable component of the second measurements is based on thevibration at a second location on the flow tube; and determining, basedon the first measurements and the second measurements, at least one of amass flow rate within the flow tube or a density of the fluid within theflow tube.

In some example methods, the first measurements are representative of afirst position of the first location on the flow tube based on detectingthe first light beam, the second measurements are representative of asecond position of the second location on the flow tube based ondetecting the second light beam. In some examples, the first location onthe flow tube is positioned at least partially between the first lightsource and the first optical sensor, and the second location on the flowtube is positioned at least partially between the second light sourceand the second optical sensor.

Some example methods involve performing fourth measurements by detectingthe first light beam or a fourth light beam, in which the fourthmeasurements include a second steady state component, and in which thecompensating of the first light output of the first light beam and thesecond light output of the second light beam involves: filtering thefirst and second steady state components; and controlling the one ormore currents to the first light source and the second light sourcebased on the filtered first and second steady state components.

In some example methods, the compensating the first light output of thefirst light beam and the second light output of the second light beaminvolves control the first light source and the second light source tooutput a substantially constant output over a range of temperatures ofthe first light source and the second light source. In some examplemethods, the first light source includes a first light emitting diode(LED) and the second light source includes a second LED, the first lightemitting diode and the second light emitting diode are coupled in seriesand have a same excitation current, and the compensating of the firstlight output of the first light beam and the second light output of thesecond light beam involves controlling the excitation current.

In some example methods, the compensating of the first light output ofthe first light beam and the second light output of the second lightbeam involves comparing the third measurements to a reference, andcontrolling the one or more currents based on the comparison. In someexample methods, the first, second, and third optical sensors arethermally coupled.

FIG. 1 is a schematic diagram of an example mass flow meter/controller100. The example mass flow meter/controller 100 of FIG. 1 may be used tomeasure mass flow and/or density of a fluid through a conduit connectedin line with the mass flow meter/controller 100, and/or to control massflow of a fluid through the conduit by controlling a valve.

The example mass flow meter/controller 100 includes a flow-through base102, a flow tube 104, a fluid inlet 106, and a fluid outlet 108. Theflow tube 104 directs a fluid from the fluid inlet 106 of the flow tube104 to the fluid outlet 108 of the flow tube 104. To measure mass flowand/or density of the fluid flowing through the flow tube 104, theexample mass flow meter/controller 100 includes multiple optical sensors110, 112 (also referred to herein as “photo sensors”), multiple lightsources 111, 113, an actuator to cause vibration in the flow tube 104(e.g., a permanent magnet 114 and a driving coil 116), and controlcircuitry 122. To reduce measurement error, the example mass flowmeter/controller 100 further includes a temperature sensor 126.

The flow tube 104 is configured in a U-shape. The driving coil 116generates an alternating magnetic field, which creates a driving forceon the permanent magnet 114, which is attached to the flow tube 104 andtransfers the driving force to the flow tube 104 to result in avibration in the flow tube 104. The flow tube 104 vibrates at afrequency, and the control circuitry 122 may control the driving coil116 to cause the vibration frequency to approximate the naturaloscillation frequency of the flow tube 104. Moving media (e.g., gas orliquid) inside the flow tube 104 creates a Coriolis force, which causesa phase shift between a first location 118 on the flow tube 104 that isupstream and a second location 120 on the flow tube 104 that isdownstream. The optical sensors 110, 112 measure the positions of theflow tube 104 at the first and second locations 118, 120 and outputrespective signals (e.g., measurements) having the same frequency, buthaving a phase or time difference. The first location 118 on the flowtube 104 is positioned at least partially between the first light source111 and the first optical sensor 110, and the second location 120 on theflow tube 104 is positioned at least partially between the second lightsource 113 and the second optical sensor 112.

The example control circuitry 122 determines a mass flow rate throughthe flow tube 104 and/or a density of the fluid in the flow tube 104based on first measurements from the optical sensor 110 and secondmeasurements from the optical sensor 112. In some examples, the controlcircuitry 122 controls a mass flow rate through the flow tube 104 usinga flow control valve 124. The control circuitry 122 may control the flowcontrol valve 124 based on a comparison of a desired flow rate and themeasured flow rate, and may include one or more control loops, such as aproportional-integral-derivative (PID) controller, and/or one or morefilters.

The example control circuitry 122 of FIG. 1 may be a general-purposecomputer, a laptop computer, a tablet computer, a mobile device, aserver, an embedded device, and/or any other type of computing device.

The example control circuitry 122 of FIG. 1 includes a processor 132.The example processor 132 may be any general purpose central processingunit (CPU) from any manufacturer. In some other examples, the processor132 may include one or more specialized processing units, such asgraphic processing units and/or digital signal processors. The processor132 executes machine readable instructions 134 that may be storedlocally at the processor (e.g., in an included cache), in a randomaccess memory 136 (or other volatile memory), in a read only memory 138(or other non-volatile memory such as FLASH memory), and/or in a massstorage device 140. The example mass storage device 140 may be a harddrive, a solid state storage drive, a hybrid drive, a RAID array, and/orany other mass data storage device.

A bus 142 enables communications between the processor 132, the RAM 136,the ROM 138, the mass storage device 140, a network interface 144,and/or an input/output interface 146.

The example network interface 144 includes hardware, firmware, and/orsoftware to connect the control circuitry 122 to a communicationsnetwork 148 such as the Internet. For example, the network interface 144may include IEEE 802.X-compliant wireless and/or wired communicationshardware for transmitting and/or receiving communications.

The example control circuitry 122 may access a non-transitory machinereadable medium 152 via the I/O interface 146 and/or the I/O device(s)150. Examples of the machine readable medium 152 of FIG. 1 includeoptical discs (e.g., compact discs (CDs), digital versatile/video discs(DVDs), Blu-ray discs, etc.), magnetic media (e.g., floppy disks),portable storage media (e.g., portable flash drives, secure digital (SD)cards, etc.), and/or any other type of removable and/or installedmachine readable media.

To determine the mass flow rate, the example control circuitry 122 mayuse the mass flow equation shown in Equation 1 below:

MF=FCF*Δt  (Equation 1)

In Equation 1, MF is the mass flow (e.g., kilograms/second (kg/s), FCFis the flow calibration factor, which is a constant for a specificdevice (e.g., based on a calibration), and

${{\Delta t} = \frac{\theta}{2\pi F}},$

in which θ is the phase difference between the output signals from theoptical sensors 110, 112, and F is the natural oscillation frequency ofthe flow tube 104.

The example light sources 111, 113 are configured to emit respectivelight beams 154, 156, and the optical sensors 110, 112 are configured tooutput measurements based on detecting the respective light beams 154,156. The vibration of the flow tube 104 modulates the light beams 154,156, which causes modulation of the measurement signals output by theoptical sensors 110, 112.

As mentioned above, the optical sensors 110, 112 (e.g., the LED(s) andthe photodiode(s) of the optical sensors 110, 112) have characteristicsthat may change as a function of temperature. A higher variability inthe output signals from the optical sensors 110, 112 due to changes intemperature can reduce the accuracy of measurements and/or control bythe mass flow meter/controller 100. To stabilize the output over a rangeof temperatures, whether short-term or long-term changes, the examplemass flow meter/controller 100 includes a compensation circuit 158coupled to the optical sensors 110, 112. The compensation circuit 158controls the light output of the light beams 154, 156 by controlling oneor more currents to the light sources 111, 113 based on a steady statecomponent(s) of one or more outputs of the optical sensors 110, 112.

The example compensation circuit 158 has the advantage of avoiding usingthe modulated output signals of the optical sensors 110, 112. Instead,the compensation circuit 158 uses one or more signals that have steadystate components representative of the light output but do not requirefiltering of modulated signals, which can be imperfect and result innoise in the light output compensation. In some examples, the lightsources 111, 113 are coupled to share an excitation current (e.g.,coupled in series) to reduce the number of variables between the lightsources 111, 113. The optical sensors 110, 112, the light sources 111,113, and the compensation circuit 158 form part of an opticalmeasurement circuit 160, as disclosed in more detail below. Exampleimplementations of the optical measurement circuit 160 and thecompensation circuit 158 are disclosed below with reference to FIGS. 2-5.

FIG. 2 is a circuit diagram of an example implementation of the opticalsensors 110, 112, the light sources 111, 113, the optical measurementsystem 160, and the compensation circuit 158 of FIG. 1 . In the exampleof FIG. 2 , the first optical sensor 110 outputs first measurements 202based on detecting the first light beam 154, which is modulated by thefirst location 118 of the flow tube 104. The second optical sensor 112outputs second measurements 204 based on detecting the second light beam156, which is modulated by the second location 120 of the flow tube 104.The first and second measurements 202, 204 may be output to the controlcircuitry 122 of FIG. 1 for determining a mass flow rate through theflow tube 104 and/or a density of the fluid in the flow tube 104 basedon the first measurements 202 and the second measurements 204.

The example optical measurement system 160 of FIG. 2 includes a thirdlight source 206 and a third optical sensor 208 configured to provide anoutput for compensating the light sources 111, 113 and/or the opticalsensors 110, 112 for variability due to temperature, aging, and/or anyother causes. The light sources 111, 113 are coupled in series and areboth coupled in series with the third light source 206, such thatcompensating for changes in the output of the third light source 206results in compensation of the light sources 111, 113.

The optical sensor 208 measures a third light beam 210 output by thelight source 206, which is not modulated by the flow tube 104. As aresult, the output of the optical sensor 208 has a steady statecomponent and does not have a substantial variable component. Instead,the steady state component is representative of the steady state outputof the light source 206, which may be assumed to be representative ofthe light output of the light sources 111, 113. In the example of FIG. 2, the light sources 111, 113, 206 share an excitation current (e.g.,forward current). In some examples, the light sources 111, 113, 206 maybe arranged to limit (e.g., reduce, minimize) any temperature gradientbetween the light sources 111, 113, 206 via thermally coupling the lightsources 111, 113, 206.

The example compensation circuit 158 further includes acomparator-amplifier 212 configured to compare the output of the opticalsensor 208 to a reference signal (e.g., a reference voltage 214), and tooutput a compensation signal 216 to an LED control circuit 218. In theexample of FIG. 2 , the compensation signal 216 is proportional to thedifference between the output of the optical sensor 208 and thereference voltage 214. The LED control circuit 218 controls theexcitation current to the light sources 111, 113, 206 based on thecompensation signal 216. The stability of the light output of the lightbeams 154, 156, 210 depends on the stability of the voltage referenceand the parameters of the comparator-amplifier 212, which may beconfigured as a PID controller and/or any other type of feedback loop.

FIG. 3 is a circuit diagram of another example implementation of theoptical sensors 110, 112, the light sources 111, 113, the opticalmeasurement system 160, and the compensation circuit 158 of FIG. 1 . Inthe example of FIG. 3 , the compensation circuit 158 includes a thirdoptical sensor 302 that is coupled to one of the light sources 111, 113.In the example of FIG. 3 , the optical sensors 110, 112, 302 arephotodiodes.

Compared to the example of FIG. 2 , the example implementation of FIG. 3reduces the number of light emitters by coupling the third opticalsensor 302 to the light source 113 (or the light source 111) such that alight beam 304 emitted by the light source 113 and measured by the thirdoptical sensor 302 is not modulated by the flow measurement tube 104.For example, the third optical sensor 302 may be coupled directly to theoutput of the light source 113, as illustrated in FIG. 5 below. Theoutput measurements of the optical sensor 302 are provided to thecomparator-amplifier 212, which provides the compensation signal 216 asdescribed above with reference to FIG. 2 .

The LED light intensity stabilization in the examples of FIGS. 2 and 3is based on an assumption that all of the optical sensors 110, 112, 208and/or 110, 112, 302 have substantially identical temperaturecharacteristics (e.g., identical within a margin of measurement accuracythat is considered acceptable for a given application). However, thereare some variations in characteristics from part to part. FIG. 4 is acircuit diagram of another example implementation of the optical sensors110, 112, the light sources 111, 113, the optical measurement system160, and the compensation circuit 158 of FIG. 1 , which may be used tofurther reduce measurement error. In the example implementation of FIG.4 , the optical measurement system 160 includes additional opticalsensors 402, 404.

The fourth optical sensor 404 is coupled to the first light source 111such that a light beam 406 emitted by the first light source 111 andmeasured by the fourth optical sensor 404 is not modulated by the flowmeasurement tube 104. Similarly, the third optical sensor 402 is coupledto the second light source 113 such that a light beam 408 emitted by thesecond light source 113 and measured by the third optical sensor 402 isalso not modulated by the flow measurement tube 104.

The fourth optical sensor 404 outputs measurements (e.g., a firststeady-state component) to a first comparator-amplifier 410 and thethird optical sensor 402 outputs measurements (e.g., a secondsteady-state component) to a second comparator-amplifier 412. Thereference voltage 214 is coupled to both of the comparator-amplifiers410, 412. The comparator-amplifier 410 outputs a compensation signal 414proportional to the difference between the measurements by the fourthoptical sensor 404 (e.g., the first steady-state components) and thereference voltage 214. Similarly, the comparator-amplifier 412 outputs acompensation signal 416 proportional to the difference between themeasurements by the third optical sensor 402 (e.g., the secondsteady-state components) and the reference voltage 214. The compensationsignals 414, 416 are input to a filter circuit 418, which filters thecompensation signals 414, 416 to output a filtered compensation signal420. In the example of FIG. 4 , the filter circuit 418 averages thecompensation signals 414, 416 to generate the filtered (e.g., averaged)compensation signal 420. However, other filter functions may be usedinstead of averaging.

The filter circuit 418 outputs the filtered compensation signal 420 tothe LED control circuit 218, which controls the excitation current tothe light sources 111, 113 (e.g., the light beams 406, 408).

FIG. 5 illustrates an example implementation of a light source 502having an optical sensor 504 coupled to receive unmodulated light 506output by the light source 502. The optical sensor 504 is attacheddirectly to the light source 502 to having a high degree of couplingbetween the light source 502 and the optical sensor 504. Additionally,the coupling of the optical sensor 504 remains consistent to reduce oreliminate variation in the incidence of light 506 emitted by the lightsource 502 on the optical sensor 504.

FIG. 6 is a flowchart representative of an example method 600 that maybe performed by the mass flow meter/controllers 100 and compensationcircuits 158 of FIGS. 1-4 to compensate optical sensors and/or lightsources for changes in temperature and/or other effects. The examplemethod 600 is described below with reference to the mass flowmeter/controller 100 of FIG. 1 and the compensation circuit 158 of FIG.3 . However, the method 600 may be performed with other devices havingmultiple optical sensors and/or different compensation circuits, such asthe compensation circuits of FIGS. 2 and/or 4 .

At block 602, the LED control circuit 218 controls the excitationcurrent to the light sources 111, 113 to emit the first light beam 154via the first light source 111 and emit the second light beam 156 viathe second light source 113. In the example of FIG. 3 , the lightsources 111, 113 are in series and have the same excitation current.

At block 604, the first optical sensor 110 performs first measurementsof the first light beam 154. For example, the first light beam 154 maybe modulated by the flow tube 104, and the modulated light beam ismeasured by the first optical sensor 110 and output to the controlcircuitry 122 of FIG. 1 .

At block 606, the second optical sensor 112 performs first measurementsof the second light beam 156. For example, the second light beam 156 maybe modulated by the flow tube 104, and the modulated light beam ismeasured by the second optical sensor 112 and output to the controlcircuitry 122.

At block 608, the third optical sensor 302 performs third measurementsof the second light beam 156 (or the first light beam 154). In contrastwith the measurements by the second optical sensor 112, the measurementsof the second light beam 156 by the third optical sensor 302 are notmodulated by the flow tube 104, and the third optical sensor 302 outputsthe measurements (e.g., a steady state component) to thecomparator-amplifier 212.

At block 610, the control circuitry 122 (e.g., via the processor 132)determines a mass flow rate through the flow tube 104 and/or a densitymeasurement based on the variable components of the first and secondmeasurements by the first and second optical sensors 110, 112. Forexample, the control circuitry 122 may calculate the mass flow ratebased on a phase difference between the first and second measurements,and/or calculate a density of the fluid based on the frequency of thevibration in the flow tube 104.

At block 612, the comparator-amplifier 212 compares the steady stateportion of the third measurements to a reference. For example, thecomparator-amplifier 212 compares the measurements from the thirdoptical sensor 302 to the reference voltage 214 to generate thecompensation signal 216. The example compensation signal 216 isproportional to a difference between the steady state portion of themeasurements from the third optical sensor 302 and the reference voltage214.

At block 614, the LED control circuit 218 adjusts the excitation currentbased on the comparison (e.g., based on the compensation signal 216).

At block 616, the control circuitry 122 determines whether the flow rateis to be controlled. For example, a mass flow controller may beconfigured to control the flow rate, while a mass flow meter omitscontrolling the flow rate. If the flow rate is to be controlled (block616), the control circuitry 122 adjusts the flow control valve 124 basedon the difference between the measured flow rate and a target flow rate.

After adjusting the flow control valve (block 618), or if the flowcontrol rate is not being controlled (block 616), control returns toblock 602 to continue measurement and/or control.

The present methods and systems may be realized in hardware, software,and/or a combination of hardware and software. The present methodsand/or systems may be realized in a centralized fashion in at least onecomputing system, or in a distributed fashion where different elementsare spread across several interconnected computing systems. Any kind ofcomputing system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may include a general-purpose computing system with a programor other code that, when being loaded and executed, controls thecomputing system such that it carries out the methods described herein.Another typical implementation may comprise one or more applicationspecific integrated circuit or chip. Some implementations may comprise anon-transitory machine-readable (e.g., computer readable) medium (e.g.,FLASH memory, optical disk, magnetic storage disk, or the like) havingstored thereon one or more lines of code executable by a machine,thereby causing the machine to perform processes as described herein. Asused herein, the term “non-transitory machine-readable medium” isdefined to include all types of machine readable storage media and toexclude propagating signals.

As utilized herein the terms “circuits” and “circuitry” refer tophysical electronic components (i.e. hardware) and any software and/orfirmware (“code”) which may configure the hardware, be executed by thehardware, and or otherwise be associated with the hardware. As usedherein, for example, a particular processor and memory may comprise afirst “circuit” when executing a first one or more lines of code and maycomprise a second “circuit” when executing a second one or more lines ofcode. As utilized herein, “and/or” means any one or more of the items inthe list joined by “and/or”. As an example, “x and/or y” means anyelement of the three-element set {(x), (y), (x, y)}. In other words, “xand/or y” means “one or both of x and y”. As another example, “x, y,and/or z” means any element of the seven-element set {(x), (y), (z), (x,y), (x, z), (y, z), (x, y, z)}. In other words, “x, y and/or z” means“one or more of x, y and z”. As utilized herein, the term “exemplary”means serving as a non-limiting example, instance, or illustration. Asutilized herein, the terms “e.g.,” and “for example” set off lists ofone or more non-limiting examples, instances, or illustrations. Asutilized herein, circuitry is “operable” to perform a function wheneverthe circuitry comprises the necessary hardware and code (if any isnecessary) to perform the function, regardless of whether performance ofthe function is disabled or not enabled (e.g., by a user-configurablesetting, factory trim, etc.).

The present methods and/or systems may be realized in hardware,software, or a combination of hardware and software. The present methodsand/or systems may be realized in a centralized fashion in at least onecomputing system, or in a distributed fashion where different elementsare spread across several interconnected computing systems. Any kind ofcomputing system or other apparatus adapted for carrying out the methodsdescribed herein is suited. A typical combination of hardware andsoftware may be a general-purpose computing system with a program orother code that, when being loaded and executed, controls the computingsystem such that it carries out the methods described herein. Anothertypical implementation may comprise an application specific integratedcircuit or chip. Some implementations may comprise a non-transitorymachine-readable (e.g., computer readable) medium (e.g., FLASH drive,optical disk, magnetic storage disk, or the like) having stored thereonone or more lines of code executable by a machine, thereby causing themachine to perform processes as described herein.

While the present method and/or system has been described with referenceto certain implementations, it will be understood by those skilled inthe art that various changes may be made and equivalents may besubstituted without departing from the scope of the present methodand/or system. In addition, many modifications may be made to adapt aparticular situation or material to the teachings of the presentdisclosure without departing from its scope. For example, blocks and/orcomponents of disclosed examples may be combined, divided, re-arranged,and/or otherwise modified. Therefore, it is intended that the presentmethod and/or system not be limited to the particular implementationsdisclosed, but that the present method and/or system will include allimplementations falling within the scope of the appended claims.

What is claimed is:
 1. A method to perform optical measurements, themethod comprising: emitting a first light beam via a first light source;performing first measurements by detecting the first light beam via afirst optical sensor; emitting a second light beam via a second lightsource; performing second measurements by detecting the second lightbeam via a second optical sensor, wherein the first measurements and thesecond measurements comprise variable components; performing thirdmeasurements by detecting a third light beam emitted from the secondlight source via a third optical sensor, wherein the third measurementscomprise a first steady state component representative of lightintensities of the first light source and the second light source; andcompensating a first light output of the first light beam and a secondlight output of the second light beam by controlling one or morecurrents to the first light source and the second light source based onthe first steady state component of the third measurements.
 2. Themethod as defined in claim 1, further comprising: directing fluid froman inlet of a flow tube to an outlet of the flow tube; causing avibration in the flow tube, wherein a first variable component of thefirst measurements is based on the vibration at a first location on theflow tube, and a second variable component of the second measurements isbased on the vibration at a second location on the flow tube; anddetermining, based on the first measurements and the secondmeasurements, at least one of a mass flow rate within the flow tube or adensity of the fluid within the flow tube.
 3. The method as defined inclaim 2, wherein the first measurements are representative of a firstposition of the first location on the flow tube based on detecting thefirst light beam, the second measurements are representative of a secondposition of the second location on the flow tube based on detecting thesecond light beam.
 4. The method as defined in claim 2, wherein thefirst location on the flow tube is positioned at least partially betweenthe first light source and the first optical sensor, and the secondlocation on the flow tube is positioned at least partially between thesecond light source and the second optical sensor.
 5. The method asdefined in claim 1, wherein the compensating the first light output ofthe first light beam and the second light output of the second lightbeam comprises controlling the first light source and the second lightsource to output a substantially constant output over a range oftemperatures of the first light source and the second light source. 6.The method as defined in claim 1, wherein the first light sourcecomprises a first light emitting diode and the second light sourcecomprises a second light emitting diode, the first light emitting diodeand the second light emitting diode are coupled in series and have asame excitation current, and the compensating of the first light outputof the first light beam and the second light output of the second lightbeam comprises controlling the excitation current.
 7. The method asdefined in claim 1, wherein the compensating of the first light outputof the first light beam and the second light output of the second lightbeam comprises comparing the third measurements to a reference, andcontrolling the one or more currents based on the comparison.
 8. Themethod as defined in claim 1, wherein the first, second, and thirdoptical sensors are thermally coupled.
 9. A method to perform opticalmeasurements, the method comprising: emitting a first light beam via afirst light source; performing first measurements by detecting the firstlight beam via a first optical sensor; emitting a second light beam viaa second light source; performing second measurements by detecting thesecond light beam via a second optical sensor, wherein the firstmeasurements and the second measurements comprise variable components;performing third measurements by detecting a third light beam emittedfrom the second light source via a third optical sensor, wherein thethird measurements comprise a first steady state componentrepresentative of light intensities of the second light source;performing fourth measurements by detecting a fourth light beam emittedfrom the first light source via a fourth optical sensor, wherein thefourth measurements comprise a second steady state componentrepresentative of light intensities of the first light source; filteringthe first and second steady state components via a filter circuit; andcontrolling, via a compensation circuit, the one or more currents to thefirst light source and the second light source based on the filteredfirst and second steady state components.
 10. The method as defined inclaim 9, further comprising: directing fluid from an inlet of a flowtube to an outlet of the flow tube; causing a vibration in the flowtube, wherein a first variable component of the first measurements isbased on the vibration at a first location on the flow tube, and asecond variable component of the second measurements is based on thevibration at a second location on the flow tube; and determining, basedon the first measurements and the second measurements, at least one of amass flow rate within the flow tube or a density of the fluid within theflow tube.
 11. The method as defined in claim 10, wherein the firstmeasurements are representative of a first position of the firstlocation on the flow tube based on detecting the first light beam, thesecond measurements are representative of a second position of thesecond location on the flow tube based on detecting the second lightbeam.
 12. The method as defined in claim 10, wherein the first locationon the flow tube is positioned at least partially between the firstlight source and the first optical sensor, and the second location onthe flow tube is positioned at least partially between the second lightsource and the second optical sensor.
 13. The method as defined in claim9, wherein the compensating the first light output of the first lightbeam and the second light output of the second light beam comprisescontrolling the first light source and the second light source to outputa substantially constant output over a range of temperatures of thefirst light source and the second light source.
 14. The method asdefined in claim 9, wherein the first light source comprises a firstlight emitting diode and the second light source comprises a secondlight emitting diode, the first light emitting diode and the secondlight emitting diode are coupled in series and have a same excitationcurrent, and the compensating of the first light output of the firstlight beam and the second light output of the second light beamcomprises controlling the excitation current.
 15. A method to performoptical measurements, the method comprising: emitting a first light beamvia a first light source; performing first measurements by detecting thefirst light beam via a first optical sensor; emitting a second lightbeam via a second light source; performing second measurements bydetecting the second light beam via a second optical sensor, wherein thefirst measurements and the second measurements comprise variablecomponents; emitting a third light beam via a third light source;performing third measurements by detecting the third light beam via athird optical sensor, wherein the third measurements comprise a firststeady state component representative of light intensities of the firstlight source and the second light source; controlling, via acompensation circuit, a first light output of the first light beam and asecond light output of the second light beam by controlling one or morecurrents to the first light source and the second light source based onthe first steady state component of the third measurements.
 16. Themethod as defined in claim 15, further comprising: directing fluid froman inlet of a flow tube to an outlet of the flow tube; causing avibration in the flow tube, wherein a first variable component of thefirst measurements is based on the vibration at a first location on theflow tube, and a second variable component of the second measurements isbased on the vibration at a second location on the flow tube; anddetermining, based on the first measurements and the secondmeasurements, at least one of a mass flow rate within the flow tube or adensity of the fluid within the flow tube.
 17. The method as defined inclaim 16, wherein the first measurements are representative of a firstposition of the first location on the flow tube based on detecting thefirst light beam, the second measurements are representative of a secondposition of the second location on the flow tube based on detecting thesecond light beam.
 18. The method as defined in claim 16, wherein thefirst location on the flow tube is positioned at least partially betweenthe first light source and the first optical sensor, and the secondlocation on the flow tube is positioned at least partially between thesecond light source and the second optical sensor.
 19. The method asdefined in claim 15, wherein the compensating the first light output ofthe first light beam and the second light output of the second lightbeam comprises controlling the first light source and the second lightsource to output a substantially constant output over a range oftemperatures of the first light source and the second light source. 20.The method as defined in claim 15, wherein the first light sourcecomprises a first light emitting diode and the second light sourcecomprises a second light emitting diode, the first light emitting diodeand the second light emitting diode are coupled in series and have asame excitation current, and the compensating of the first light outputof the first light beam and the second light output of the second lightbeam comprises controlling the excitation current.